1. Field of the Invention
This invention relates to a power amplifier having an ESD protection circuit and formed on a semiconductor chip based on bipolar transistors.
2. Background Art
In recent years, there is a growing demand for downsizing on compact information devices as typified by mobile phones. Small peripheral components (such as chip capacitors and inductors) are incorporated and modularized into corresponding key components, and the number of peripheral components is being reduced. Thus such modules are directly exposed to the external environment. What particularly matters in this situation is the tolerance to ESD (electrostatic discharge). ESD is a phenomenon in which electric charge accumulated in a device or human body external to a circuit is instantaneously discharged to the circuit. When a circuit is installed in a device or is in use, electric charge is inevitably produced due to the motion of the device or human body. Therefore, in a device exposed to the external environment, it is necessary to ensure ESD tolerance on the circuit side.
The main cause of ESD breakdown is believed to be thermal breakdown due to the instantaneous flow of electric charge accumulated at high energy (high potential) into the circuit. Thus the ESD protection circuit serves to prevent the high-energy electric charge from flowing into the thermally weak semiconductor device.
Conventionally, a circuit having such function is realized by a circuit having a diode connected in series between a terminal to be protected and a ground terminal for passing current when a prescribed voltage is exceeded. When a voltage above the turn-on voltage of the diode is applied to its terminal, the diode becomes conducting to prevent electric charge from flowing into the circuit to be protected. Here, the prescribed voltage needs to be set to a value higher than voltages required for operation of the circuit such as the power supply voltage and the operating voltage. To this end, a plurality of stages of diodes are used as disclosed in JP 2003-023084A.
However, this type of circuit has a problem that the diode itself constituting the protection circuit is broken by the inflow of high energy. To prevent this, it may be contemplated as disclosed in JP 8-236637A that a resistor is inserted in series with the protection device (such as a diode) to cause voltage drop and reduce current flowing into the protection device. In this case, the voltage drop increases as the value of resistance increases, and thereby the tolerance is improved. However, because energy inflow to the protection circuit side is disturbed, the operation as a protection circuit becomes uncertain. Thus a resistor with several to ten and several ohms is required. Furthermore, because the resistor itself must withstand large energy loss, it is necessary to use a resistor having a high current capacity realized by a semiconductor rather than a resistor having a small current capacity realized by a metal thin film.
However, the value of sheet resistance of a semiconductor resistor is ten and several Ω/□ even with high-concentration doping. To achieve several Ω requires a large aspect ratio, which consumes a large area on the chip. Then the chip area is increased, and the increase of packaging area adversely affects downsizing. Furthermore, when the wafer price is high as in the case of compound semiconductors, there is also a problem of cost.
As described above, conventional diode-connected circuits require a large area for achieving high ESD tolerance and have problems in pellet size and cost.
On the other hand, JP 2001-274636A discloses an amplifier which is excellent in uniformity of current distribution in spite of a small ballast resistance and has high efficiency and low distortion, exhibiting little deterioration of distortion even when a digital modulation wave is input thereto. This amplifier is a high frequency power amplifier based on bipolar transistors, comprising at least two or more blocks. Each block includes a bias generation circuit for generating a base bias potential and a capacitor device for high frequency input. Each bias generation circuit comprises a second bipolar transistor for impedance conversion and a diode circuit having a current mirror transistor, the diode circuit being subjected to constant current biasing for temperature detection.